Hydrogen-resistant optical fiber/grating structure suitable for use in downhole sensor applications

ABSTRACT

A hydrogen-resistant optical fiber particularly well-suitable for downhole applications comprises a relatively thick pure silica core and a depressed-index cladding layer. Interposed between the depressed-index cladding layer and the core is a relatively thin germanium-doped interface. By maintaining a proper relationship between the pure silica core diameter and the thickness of the germanium-doped interface, a majority (preferably, more than 65%) of the propagating signal can be confined within the pure silica core and, therefore, be protected from hydrogen-induced attenuation problems associated with the presence of germanium (as is common in downhole fiber applications). The hydrogen-resistant fiber of the present invention can be formed to include one or more Bragg gratings within the germanium-doped interface, useful for sensing applications.

TECHNICAL FIELD

The present invention relates to a hydrogen-resistant optical fiberincluding a germanium-doped core/cladding interface that provides for arelatively stable silica glass matrix in the presence of hydrogen.

BACKGROUND OF THE INVENTION

One of the niche applications for fiber optics is as a sensor for“downhole” applications, such as monitoring a geothermal well, oil well,or the like. Downhole measurements permit the operator to monitormultiphase fluid flow, as well as pressure and temperature. Downholemeasurements of pressure, temperature and fluid flow play an importantrole in managing various types of sub-surface reservoirs.

Historically, the monitoring systems have been configured to provide anelectrical line that allows the measuring instruments, or sensors, tosend measurements to the surface. Recently, fiber optic sensors havebeen developed that communicate readings from a wellbore to opticalsignal processing equipment located at the surface. The fiber opticsensors may be variably located within the wellbore. For example,optical sensors may be positioned on the outer surface of a submersibleelectrical pump and used to monitor the performance of the pump. Fiberoptic sensors may also be disposed along the tubing within a wellbore.In either instance, a fiber optic cable is run from the surface to thedownhole sensing apparatus. The fiber optic cable transmits opticalsignals to an optical signal processor at the surface which is then usedto determine environmental information (such as temperature and/orpressure) associated with the wellbore.

With respect to geothermal wells, a fiber optic sensor may be used toobtain a temperature profile along the depth of the well. It is wellknown in the art that a vertical temperature profile of an entiregeothermal well can be obtained essentially instantaneously using asingle optical fiber. Inasmuch as the intensity of various frequencycomponents of backscattered light within the optical fiber depend uponthe temperature of the medium at the point where the backscattered lightis generated, proper detection and analysis of the entire backscatteredradiation spectrum will yield the desired temperature profile.

However, field tests of optical fiber distributed temperature sensorshave demonstrated that conventional optical fibers are insufficientlyrobust for this type of application. In “hot” well studies, anomaliesassociated with changes in the optical transmission characteristics ofthe studied optical fibers began to appear within the first twenty-fourhours of the test period. Inasmuch as it is desired to deploy theseoptical fiber sensors for long periods of time, this type of change isunacceptable.

At least a portion of the anomalies have been associated with theformation of OH ions (and other hydrogen-related moieties) in thesilicate glass matrix of the optical fibers. The OH ions do not exist inthe optical fiber prior to its exposure to the “downhole” environment.The likely degradation mechanism is that hydrogen in the hot downholeenvironment diffuses into the fiber, and within the fiber the hydrogenreacts with the oxygen of the silicate glass to form OH ions.

The constituents of the glass have been found to have a strong influenceon the rate at which OH ions are formed in a typical downholeenvironment. Optical fibers typically have a core glass with arefractive index value that is greater than the refractive index valueof a surrounding cladding glass, so as to maintain confinement of thepropagating optical signal within the core area. An optical fiber mayhave what is referred to as a “step-index” structure, where there isessentially an abrupt interface between the core and cladding glasses,or alternatively, a “graded-index” structure, where there is a gradualchange in refractive index in a radial direction from the center of thecore. It is common, in either case, to introduce germanium into the corearea to increase its refractive index. It has been found, however, thatthe presence of germanium promotes the formation of OH ions in thedownhole environment.

Thus, a need remains in the art for an effective optical fiber sensorfor downhole applications that remains stable within a hydrogen-richenvironment, even at elevated temperatures.

SUMMARY OF THE INVENTION

The need remaining in the art is addressed by the present invention,which relates to a hydrogen-resistant optical fiber includinggermanium-doped core/cladding interface that provides for a relativelystable silica glass matrix in the presence of hydrogen.

In accordance with the present invention, the central portion of thecore structure is “germanium-free”, pure silica. As such, there islittle or no opportunity for OH ions to be created. The germanium-dopedcore/cladding interface region is formed to comprise only a smallfraction of the core diameter and, therefore, interacts with only arelatively small percentage of the fundamental signal mode propagatingthrough the core (for example, less than 90% of the fundamental mode).This limited interaction is sufficient to significantly minimize thedeleterious effects of hydrogen-induced attenuation. Any hydrogen thatis present will react with the germanium present in the core/claddinginterface and thus remain separated from the pure silica interior regionof the core. A fluorine-doped cladding layer is formed to surround thegermanium-doped core/cladding interface.

In a preferred embodiment of the present invention, thehydrogen-resistant fiber comprises a relatively thick pure silica core,with several separate layers of germanium-doped glass formed to surroundthe pure silica core. Inasmuch as the core is relatively thick, only asmall portion of germanium dopant will migrate into the core region.

In a preferred method of making the hydrogen-resistant fiber of thepresent invention, an MCVD technique is used, starting with thedeposition of the fluorine-doped depressed cladding layer, followed bythe deposition of the several germanium-doped layers, with a pure silicacore layer deposited thereafter. The preform tube is then collapsed toform the solid core region, with the surrounding germanium-dopedcore/cladding interface and the fluorine-doped depressed cladding layersurrounding the interface.

In a further embodiment of the present invention, a Bragg grating may bewritten in the fiber as it is being drawn, where the grating will beformed in the interface region (requiring the presence of germanium toform the grating structure). There are many downhole sensor applicationsthat utilize the reflective properties of a Bragg grating to measuretemperature and/or pressure along the depth of the wellbore.

Other and further embodiments and aspects of the present invention willbecome apparent during the course of the following discussion and byreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings,

FIG. 1 is a cut-away side view of a hydrogen-resistant optical fiberformed in accordance with the present invention;

FIG. 2 is a graph of the refractive index profile for thehydrogen-resistant fiber of FIG. 1;

FIG. 3 contains a plot comparing the hydrogen-induced attenuation forthe fiber of the present invention to various prior art fibercompositions;

FIG. 4 is a flowchart of an exemplary process for forming thehydrogen-resistant fiber of the present invention;

FIG. 5 is a flowchart of an alternative process for forming thehydrogen-resistant fiber of the present invention;

FIG. 6 is an isometric cross-sectional view of the hydrogen-resistantfiber of the present invention formed to include at least one Bragggrating along the germanium-doped interface thereof;

FIG. 7 illustrates an exemplary draw tower arrangement for forming anoptical fiber from a preform, the illustration including a UV radiationsource used to form a Bragg grating as the fiber is being drawn;

FIG. 8 is a plot of the hydrogen-induced attenuation associated with afiber Bragg grating structure formed in accordance with the presentinvention;

FIG. 9 contains two optical backscatter output plots, where FIG. 9( a)is a plot of a plurality of gratings formed along a length ofhydrogen-resistant fiber of the present invention and FIG. 9( b) is aplot of the frequency response for an exemplary grating; and

FIG. 10 is a plot illustrating the changes in hydrogen-inducedattenuation as a function of time for the inventive fiber of the presentinvention, illustration the saturation of the loss at a value ofapproximately 15 dB/km.

DETAILED DESCRIPTION

FIG. 1 contains a cross-sectional view of an exemplaryhydrogen-resistant optical fiber 10 for downhole sensor applicationsformed in accordance with the present invention. As shown in FIG. 1,fiber 10 comprises a pure silica core 12, a surrounding germanium-dopedcore/cladding interface region 14, a fluorine-doped depressed claddinglayer 16 and an outer cladding layer 18. In accordance with the presentinvention, Ge-doped interface region 14 is relatively thin, with silicacore 12 comprising the majority of the core area (preferably, over 65%of the core comprises pure silica). Therefore, the majority of theoptical signal will be propagating within the pure silica core regionand not be affected by any hydrogen-associated losses within Ge-dopedinterface region 14, where only a “tail” portion of the optical energywill reside. Interface region 14 may also be doped with, for example,one of Sb, Hf, Ta, P, Al, S, Bi, Pb, In, Ga and La.

The fluorine dopant in cladding layer 16 is used to decrease therefractive index value of fiber 10 with respect to the refractive indexvalue of core 12. The refractive index value of outer cladding layer 18is greater than that of depressed cladding layer 16. FIG. 2 contains agraph of the refractive index of hydrogen-resistant fiber 10, measuredoutward in a radial direction from the center C of core 12. Thedifference in refractive index between silica core 12/Ge-doped interfaceregion 14 and F-doped depressed cladding layer 16 is denoted as “Δ” inFIG. 2.

FIG. 3 contains a graph of hydrogen-induced attenuation, comparing theattenuation values of inventive fiber 10 with two different prior artfibers, a conventional downhole sensor fiber including a Bragg grating(denoted as “sensor” in the graph legend) and a “defect-free” puresilica core single mode fiber. For various reasons unrelated to hydrogensensitivity, “defect-free” pure silica core single mode fiber is notsuitable for downhole sensor applications. As shown, the attenuationachieved with inventive fiber 10 is significantly improved over thatassociated with the conventional Bragg grating sensor fibers,approaching the optimal value associated with the “defect-free” puresilica core single mode fiber. Indeed, the attenuation associated withinventive fiber 10 maintains a value well below 2 dB/km at mostsensor-associated wavelengths (e.g., 1300-1600 nm). Inasmuch as thegermanium-doped interface comprises only a minimal amount of the corearea (no more than 35%, and preferably even less), the majority of theoptical signal will propagate unaffected within the pure silica coreregion.

Optical fibers, including inventive hydrogen-resistant fiber 10 of thepresent invention, are generally made by chemical processes. Aparticularly useful process is known in the art as modified chemicalvapor deposition (MCVD), and FIG. 4 contains a flowchart of an exemplaryset of MCVD process steps that may be used to form hydrogen-resistantfiber 10 of the present invention. Referring to FIG. 4, the processbegins at step 100 with fluorine-doped SiO₂ being deposited within aglass tube (where the glass tube will eventually form outer claddinglayer 18 of fiber 10). In one particular MCVD process, a plurality ofseparate gases are flowed through glass tube so as to form multiplelayers of fluorine-doped glass. In order to form a relatively thickdepressed cladding layer (see FIGS. 1 and 2), a large number of layersare formed. In one exemplary embodiment, thirty to sixty separateF-doped layers may be deposited on the inner wall of a glass tube (othersuitable numbers of layers being possible, as a function of thedimensions of the deposition tube, desired fiber geometry, and thelike). Table I includes the particular gasses and flow rates associatedwith forming F-doped depressed cladding layer 16.

TABLE I Gas Flow Rate (ml/min) SiCl₄ 1000 SiF₄ 1000 O₂ 1000 He 400

The exemplary MCVD process then continues at step 110 by depositing thegermanium-doped layers forming Ge-doped interface region 14 on theexposed surface of the deposited F-doped material within the tube. Inorder to maintain this region relatively thin, only a few layers aredeposited, where three layers have been found to be suitable for mostapplications. Table II includes the particular gasses and flow ratesassociated with forming Ge-doped interface region 14.

TABLE II Gas Flow Rate (ml/min) SiCl₄ 45 GeCl₄ 90 O₂ 45 He 250

Following the deposition of the Ge-doped interface region, the core areaof inventive fiber 10 is formed by depositing a single layer of silica(shown as step 120 in FIG. 4). In accordance with the present invention,the pure silica core layer is deposited to comprise a thicknesssignificantly greater than the Ge-doped layers (as shown in FIG. 1 inparticular). Table III includes the particular gasses and flow ratesassociated with forming pure silica core 12.

TABLE III Gas Flow Rate (ml/min) SiCl₄ 260 O₂ 450 He 250

The final step in the formation of an optical fiber “preform” using anMCVD process, shown as step 130 in FIG. 4, is to collapse the tube(using a heat process, for example) to form a solid core preform fromwhich an optical fiber may then be drawn down in conventional fashion.

It is believed that the germanium within interface region 14 diffusesinward toward the center of core region 12, and also outward intofluorine-doped cladding region 16. It is further envisioned that aportion of the fluorine diffuses into interface region 14, reducing itsrefractive index to a value close to that of pure silica. As a result,interface region 14 becomes indistinguishable from pure silica coreregion 12 in terms of its refractive index value.

FIG. 5 contains a flowchart of an alternative MCVD process that may beused to form inventive fiber 10 of the present invention. In this,initial steps 100 and 110 remain the same, with the deposition of thepure silica core material in step 120 replaced by a three-step processincluding: (1) depositing silica soot (step 122), (2) treating the sootin a gaseous solution of SiCl₄ with minimal or no oxygen (step 124), and(3) sintering the saturated soot to form the pure silica material (step126). The final step in the process is the same as that shown in theflowchart of FIG. 4, to collapse the MCVD-produced tube into a solidcore preform (step 130).

While up to this point, the description of the present invention hasconcentrated on the composition and fabrication process of relativelystable hydrogen-resistant fiber, a fiber Bragg grating may also beformed within the same structure, where the grating is “written” intothe Ge-doped interface region 14. Fiber Bragg gratings are of particularinterest in downhole applications as a distributed sensor. Fiber opticBragg grating sensors have been used to measure the longitudinal andtransverse strain, as well as longitudinal strain and temperature. Inparticular, changes in ambient temperature will result in shifting thereflective wavelength of the grating structure in a known fashion.Therefore, by monitoring the reflected wavelength, the downholetemperature may be measured. Changes in pressure induce a differentmodification/shift of the center wavelength of a Bragg grating and isanother useful downhole measurement.

FIG. 6 contains a cut-away isometric view of hydrogen-resistant fiber 10of the present invention include a Bragg grating structure 20 formedwithin Ge-doped interface region 14. As is well-known in the art, aBragg structure may be formed along a predetermined length of a sectionof optical fiber by using a controlled UV exposure that functions toalter the refractive index of the fiber in a periodic fashion. In thiscase, Bragg grating structure 20 is formed to comprise a grating perioddenoted “Λ”. It is to be understood that multiple Bragg gratings ofdiffering periodicities may be written in the same fiber (eitherphysically overlapping or sequentially formed along the fiber) toperform a number of different measurements.

It is an advantage of the structure of the inventive fiber that theBragg grating is formed in the relatively thin Ge-doped interface region14, where a sufficient energy of propagating optical signal is presentto perform the monitoring function without interrupting the propagatingof the majority of the optical signal along core 12.

In one exemplary process, Bragg grating structures 20 may be formedwithin Ge-doped interface region 14 as the optical is being drawn downfrom the solid core preform. FIG. 7 illustrates, in a simplified view,an exemplary draw tower 50, where a solid core preform 52 is firstpassed through a high temperature furnace 54 to “melt” the preform andallow a glass fiber to be drawn. The drawn fiber then follows a downwardpath to a capstan 56 and take-up spool 58, where the tension/pullassociated with capstan 56 and take-up spool 58 (as well as their speed)controls the drawing process. In accordance with the present invention,a UV source 60 is disposed at a predetermined position along thedownward path of the drawing fiber to allow for the desired gratingpattern to be “written” in Ge-doped interface region 14 as the fiberpasses through source 60. Advantageously, the UV radiation will passunimpeded through F-doped depressed cladding 16, allowing for thegrating to form only in Ge-doped region 14.

FIG. 8 contains a graph of the attenuation associated with a fiber Bragggrating structure formed in accordance with the present invention andaged in a hydrogen ambient at 200° C. over a period of ninety days. Themeasurements are associated with two different Bragg gratings. As shown,the values are less then 10 dB/km in the 1500-1600 nm range. Theexperimental fiber comprises 1000 gratings of 5 mm in length, with aspacing of one meter between gratings. FIG. 9 illustrates output plotsfrom an optical backscatter reflectometer, showing in plot (a) aplurality of gratings formed along a length of about three meters. Plot(b) shows, in particular, the frequency response associated with anexemplary grating formed in the hydrogen-resistant optical fiber formedin accordance with the present invention. FIG. 10 illustrates a plot ofthe changes in hydrogen resistance as a function of time for aninventive Bragg fiber grating formed in accordance with the presentinvention. As mentioned above, downhole applications require for thefiber characteristics to remain stable over a long period of time, wherehydrogen-induced losses were seen to increase in a conventional fiberover just a twenty-four hour time period. Referring to FIG. 10, thehydrogen-induced losses in the 1500-1600 nm wavelength range are plottedfor an ambient temperature of 200° C. over a period of fifty-five days.It can be seen that losses do increase initially (over a forty dayperiod of time), but then saturate to a value of approximately 15 dB/kmand remain constant thereafter.

Rayleigh backscattered light can also be used to measure strain andtemperature by monitoring the spectrum of the backscattered light fromthe Bragg grating. This can be done with an optical frequency domainreflectometry (OFDR) system. While the OFDR system is very sensitive andcan monitor very weak signals, the use of the hydrogen-resistant fiberfor Rayleigh backscatter measurements will improve the signal levels byproviding slightly more scattering and lower losses at temperature andwith hydrogen ingress. The hydrogen-resistant fiber of the presentinvention is also advantageous for monitoring the Raman and Brilliounbackscattered light for OTDR-based distributed temperature and strainsystems in high temperature and hydrogen-rich environments.

Although the invention has been described by way of exemplaryembodiments, it should be understood that many changes and substitutionsmay be made by those skilled in the art without departing from thespirit and the scope of the invention which is defined only by theappended claims.

1. A hydrogen-resistant optical fiber comprising a pure silica coreexhibiting a known refractive index and having a substantially largediameter; a depressed-index cladding layer disposed to surround the puresilica core, the depressed-index cladding layer including a refractiveindex-lowering dopant; and a germanium-doped interface region disposedbetween the pure silica core and the depressed-index cladding layer andhaving a refractive index greater than the pure silica core, wherein thethickness of the germanium-doped interface region is less than thediameter of the pure silica core such that at least 65% of a propagatingsignal's optical energy is confined within the pure silica core and isnot affected by the intrusion of hydrogen into the germanium-dopedinterface region.
 2. The hydrogen-resistant optical fiber as defined inclaim 1 wherein the fiber further comprises an outer cladding layersurrounding the depressed-index cladding layer.
 3. Thehydrogen-resistant optical fiber as defined in claim 1 wherein thedepressed-index cladding layer is substantially thicker than thegermanium-doped interface region.
 4. The hydrogen-resistant opticalfiber as defined in claim 1 wherein the depressed-index cladding layercomprises a fluorine-doped silica glass.
 5. The hydrogen-resistantoptical fiber as defined in claim 1 wherein the depressed-index claddinglayer comprises a boron-doped silica glass.
 6. The hydrogen-resistantoptical fiber as defined in claim 1 wherein the germanium-dopedinterface further comprises an element selected from the groupconsisting of: Sb, Hf, Ta, P, Al, S Bi, Pb, In, Ga and La.
 7. Thehydrogen-resistant optical fiber as defined in claim 1 wherein theoptical fiber includes at least one Bragg grating formed along apredetermined length of the fiber within the germanium-doped interfaceregion.
 8. The hydrogen-resistant optical fiber as defined in claim 7wherein the at least one Bragg grating comprises a plurality of Bragggratings, each grating having different characteristics.
 9. Thehydrogen-resistant optical fiber as defined in claim 7 wherein the atleast one Bragg grating is configured to monitor Raleigh backscatteringreflections along the length of the optical fiber.